Polyethylene pipes are light in weight, easy to handle, and are non-corrosive. In addition, their rigidity is relatively high that they can be laid under the ground, and their flexibility is also relatively high that they can follow a movement of ground. Due to these advantageous characteristics, the amount of polyethylene pipes used is rapidly increasing in recent years.
In addition to the above desirable characteristics, polyethylene pipes should have (1) impact resistance sufficient to endure impacts given at the time when and after they are set; and (2) excellent long-term durability under gas or water pressure (specifically, environmental stress cracking resistance and internal pressure creep resistance).
With respect to the long-term durability, conventional pipes may meet the ISO standard, i.e. 50-year durability at normal temperatures under an internal pressure, expressed in terms of circumferential stress, of approximately 8 MPa. However, the conventional polyethylene pipes are still insufficient in the long-term durability for use under more severe conditions, such as main pipes for gases or running water which have a large diameter and undergo high internal pressure. For this reason, they are presently used only for branch pipes and the like, having a small diameter.
The long-term durability of a polyethylene pipe is considered to be determined by the environmental stress cracking resistance, that is the resistance to cracking which is caused when an internal pressure applied to the pipe acts as a tensile stress in the circumferential direction on the pipe over a long period of time. Therefore, in order to improve the long-term durability of polyethylene pipes, it is necessary to improve the environmental (tensile) stress cracking resistance.
For plastic pipe applications, circumferential (hoop) stress performance as set forth in ISO 9080 and ISO 1167 is an important requirement. These procedures describe the long-term creep rupture behavior of plastic materials by an extrapolation methodology wherein the hydrostatic strength of pipe materials over 50 years at 20° C. are predicted. Typically, for long term predictive performance testing, candidate pipe materials are placed at various stresses and the lifetime at a given temperature is determined. For extrapolations to 50 years at 20° C., testing is also performed at higher temperatures. The measured lifetime curves at each temperature typically consists of either a high stress, lower lifetime ductile failure mode or a lower stress, longer lifetime brittle failure mode. The ductile failure mode is referred to as Stage I failure and conversely the brittle failure mode is referred to as Stage II failure.
First and second generation polyethylene pipes for water and gas distribution have minimum required strength (MRS) ratings for respective hoop stresses of 6.3 and 8 MPa and are known as PE63 and PE80, respectively. Third generation polyethylene pipes, which are known as PE100 pipes, conform to a MRS rating of 10. The MRS rating is based on the above ISO procedures wherein a MRS rating of 10 specifies that pipes made from the polyethylene materials must withstand 10 MPa at 20° C. for 50 years.
Another important pipe or durable material performance requirement is resistance to rapid crack propagation (RCP). The RCP of a pipe material is typically measured by testing extruded pipe in accordance with ISO 13477 (the so-called ‘S4 ’ test). But the S4 test is not susceptible to small scale evaluation and as such various small scale tests have been introduced in the plastic pipe industry. Small scale testing includes the inverted Charpy test and the Plane High-Speed Double Torsion test, as well as ranking tests such as a critical strain energy release rate test or Gc measurement on compression molded materials. Also, the lower the ductile to brittle transition temperature, Tdb, of a material, the better is its RCP resistance.
In order to improve the environmental stress cracking resistance of a polyethylene composition, it is known to increase the molecular weight or to decrease the density of the polyethylene. However, when the molecular weight is increased, the fluidity of the polyethylene is lowered, so that the molding properties such as pipe-extrusion properties and injection moldability are impaired. When the density is decreased, the rigidity of the polyethylene is unfavorably lowered.
Although numerous pipe compositions have been known and used, there continues to exist a need for improved durable materials, especially for transmission and distribution pipe service for gases and water. Preferably, the materials should exhibit improved durability and/or higher temperature service lives. In particular, there is still a need for high density polyethylene durable materials with better resistance to slow crack propagation and/or rapid crack propagation.